Aluminum smelters in 1988 consumed approximately 2.5% of the electrical energy generated in the United States, or about 65,000 GW h, of which 14,000 GW h was consumed by smelters using the Soderberg technology. Domestic capacity by Soderberg aluminum smelters in 1988 was 853,000 metric tons, or 21.6% of total domestic capacity.
The Soderberg system is a continuous method for feeding anode carbon. The anode is formed within a rectangular steel shell by the sintering of a loose mixture of petroleum coke and coal tar pitch added to the top. Steel studs conduct electricity to the anode of the cell and also function to support the anode, which is the upper electrode of the cell. A typical 105,000-A Soderberg cell requires 46 to 50 studs. The present state of the art is to remove 15 to 20 studs per week from the anode of an operating aluminum reduction cell, partially fill the resulting holes with a mixture of coal tar pitch and coke, then reset freshly sand or grit blasted studs into the holes at a higher level. The temperature of the tip of the studs is about 900.degree. C. when they are pulled, and they are reset into an environment of about 650.degree. C. At these high temperatures, sulfur and carbon in the coke and pitch react with the steel studs to form a brittle crystalline layer of corrosion product that is principally iron sulfide. The layer of corrosion product is 0.1 to 0.2 inches thick on the surface of the stud at the tip where the temperature is the highest. This layer of corrosion product increases the electrical resistance of the stud and increases the voltage drop across the cell. The steel studs adhere strongly to the carbon of the anode and require a severe twist to disengage from the carbon. The act of disengagement leaves part of the corrosion product within the anode. The state of the art does nothing to mitigate corrosion of the studs, but the remaining corrosion product is removed by sand or grit blasting prior to resetting the studs at a higher level in the anode. The corrosion product left within the anode ultimately reports to and degrades the product metal as the anode is consumed. Approximately 20% of the iron impurity in product aluminum is traceable to the steel studs.
Those concerned with these and other problems recognize the need for an improved process for mitigating corrosion and increasing the conductivity of steel studs in Soderberg anodes of aluminum reduction cells.